The present invention relates to a process for desulphurising crude iron in a ladle containing the hot metal, in particular in a torpedo ladle, having a submerged lance, wherein first aluminum is added and then, by means of the carrier gas, lime is blown into the hot metal.
The blowing of fine-grain desulphurising agents into hot metal has gained increasing importance in recent years. In this process, mainly agents based on calcium carbide and magnesium are used. Although these desulphurising agents are highly effective, they are relatively expensive.
Since lime is a relatively cheap agent, desulphurising agents based on lime have therefore also been proposed. They have the disadvantage, however, that large quantities are necessary in order to obtain the desired desulphurisation effect. In practice, this leads to considerable difficulties because of the resulting large quantities of slag. The considerable volume of the large quantities of slag in the ladle causes a corresponding reduction in the transport capacity of the ladle. Moreover, the slags tend to deposit on the ladle walls, whereby the transport capacity is yet further restricted. In addition, the slags contain large quantities of iron droplets, which leads to considerable losses of iron.
The desulphurising agent which consists mainly of lime and, in addition, also proportions of 10-40% by weight of alkaline earth metal carbonate or hydroxide, 2-20% by weight of carbon, 2-10% by weight of at least one fluoride selected from the group comprising alkali metal fluorides, alkaline earth metal fluorides, cryolite and sodium silicofluoride, and 0.015-1.0% by weight of a silicone oil surface-active agent, is known from German Patent Specification No. 3.004.973.
In a further development of the use of lime as a desulphurising agent, aluminum is admixed to the desulphurising agent (German Auslegeschrift No. 2.531.047) or first aluminum and then lime are blown into the hot metal by means of high rates of conveying gas. The high rates of a non-oxidising gas, such as, for example, nitrogen, for conveying the lime also serve for dispersing the lime particles in the hot metal and for generating a circulation in the bath. However, this causes considerable disadvantages since, due to incomplete dispersion, the lime cannot deploy its full effect and considerable splashing of the hot metal occurs due to the high conveying gas rates (SDS process of Nippon Steel Corporation). Extensive progressive wear of the refractory lining of the ladle in the inlet region must also be taken into account.
In known desulphurisation processes which operate with lime as the desulphurising agent, the oxygen formed by the reaction CaO+S=CaS+O is bound by the silicon of the hot metal to give SiO.sub.2. With the lime blown in, this silica forms dicalcium silicate which surrounds the lime grain with a solid layer. As a result, the desulphurisation effect of the lime grain is adversely effected.
In the known desulphurisation processes with lime, wherein aluminum is additionally blown in to the melt, the oxygen formed by the desulphurisation reaction CaO+S is bound by the aluminum particles blown into the melt, to give Al.sub.2 O.sub.3. The alumina formed is bound by the lime blown in, to give a layer of calcium aluminates n.CaO.Al.sub.2 O.sub.3. In contrast to the solid dicalcium silicate, this layer is predominatly in liquid state above 1350.degree. C., having a good desulphurising capacity.
It was the object of the present invention to reduce both the required rate of conveying gas and the consumption of lime-containing desulphurising agent.